1. Field of the Invention
This invention relates to an exhaust purification system for an internal combustion engine, and, more particularly, to an engine exhaust purification system in which deterioration of a catalyst can be detected with a high accuracy and reliability.
2. Description of Related Art
Typically, engine exhaust purification systems have catalytic converters, such as a catalytic converter using rhodium, located in exhaust lines. By means of chemisorption action of such catalyst, unburned contaminants are removed from exhaust gases. In this type of engine exhaust purification system, the functional deterioration of catalyst is manifest as a direct reduction of its exhaust purifying efficiency. This makes it necessary to adequately monitor the functional deterioration of catalyst. From a graphically demonstrated relationship, between oxygen occluding performance or efficiency and hydrocarbon (HC) purification ratio of an end catalytic converter using rhodium shown in FIG. 15, it is apparently proved that a decreases in oxygen occludinq efficiency of the catalytic converter using rhodium is accompanied by a decrease in hydrocarbon (HC) purification ratio. In a technique of detecting the functional deterioration of this type of catalytic converter which has been developed from this peculiar characteristic or relationship, oxygen (O.sub.2) sensors are installed in the exhaust line upstream and downstream from the catalytic converter, respectively, and, based on the ratio of a reversal frequency of an output from the downstream oxygen (O.sub.2) sensor to a reversal frequency of an output from the upstream oxygen (O.sub.2) sensor, feedback control of air-fuel ratio is exercised. An example of such an engine exhaust purification system is found in, for instance, Japanese Unexamined Patent Publication No. 63-97852.
The detection of functional deterioration of a catalyst, based on the reversal frequency ratio of an output from the downstream oxygen (O.sub.2) sensor to an output from the downstream oxygen (O.sub.2) sensor, is grounded on the fact that when the catalyst keeps a normal oxygen occlusion efficiency, i.e. when it is within an allowable extent of functional deterioration, the concentration of oxygen in engine exhaust gases is less downstream from the catalytic converter than upstream from the catalytic converter, and, accordingly, the number of reversals or reversal frequency (Nb) of an output from the downstream oxygen sensor is lower than the number of reversals or reversal frequency (Na) of an output from the upstream oxygen sensor. From this fact, as shown in FIG. 16, it is said that the progress of functional deterioration of a catalyst, i.e. the deterioration of hydrocarbon (HC) purification ratio of a catalyst is accompanied by an increase in reversal frequency ratio (Nb/Na) of outputs from the downstream and upstream oxygen (O.sub.2) sensors.
Otherwise, the detection of functional deterioration of a catalyst can be made based on the amplitude of an output of the downstream oxygen (O.sub.2) sensor during a feedback control of air-fuel ratio exercised based on the output of the upstream (O.sub.2) oxygen sensor, or based on an average output of the downstream oxygen sensor, or based on the number of reversals or reversal frequency with respect to a normal threshold value of an output from the downstream oxygen sensor.
In order to assure the practical accuracy and reliability of functional deterioration detection of catalyst made on the basis of an output of an oxygen sensor or outputs of upstream and downstream oxygen sensors, the oxygen sensor must hold its sensitivity to oxygen at a desirable level. If the oxygen sensor is functionally deteriorated, the functional deterioration detection of catalyst with a high accuracy and reliability is quite difficult. The oxygen sensor exhibits two types of functional deterioration, i.e. a reduction or drop in output voltage and a delay in responsiveness. As shown in FIG. 12, portraying a drop in output voltage accompanying deterioration of the oxygen sensor, when the oxygen sensor has no functional deterioration, it provides an output of high voltage for an enriched fuel mixture, as shown by a solid line. However, following functional deterioration of the oxygen sensor, its output voltage is greatly reduced, as shown by a broken line. In considering the relationship between an output voltage of the oxygen sensor and the air-fuel ratio of a fuel mixture, as shown in FIG. 13, the oxygen sensor, providing an output at a normal threshold voltage of approximately 0.45 V for an ideal air-fuel ratio of 1 when it is functioning normally decreases or drops its output voltage to approximately 0.3 V, lower than the normal threshold voltage, even for the ideal air-fuel ratio of 1 when it exhibits functional deterioration. This output voltage drop leads to a misjudgement that the fuel mixture is lean in spite of the fact that it is still rich. As a result, a drop in output voltage of the downstream oxygen sensor due to functional deterioration causes the output voltage to reverse frequently with respect to the normal threshold voltage for the ideal air-fuel ratio of 1, i.e. to increase the number of reversals or reversal frequency. When in fact the downstream oxygen sensor changes its output characteristics in such a way, the reversal frequency ratio (Nb/Na) of an output of the downstream oxygen sensor to an output of the upstream oxygen sensor varies to a greater degree. Accordingly, notwithstanding the fact that there is no functional deterioration of a catalyst, an erroneous judgement that the catalyst has deteriorated may be falsely induced.
Conversely, if there is a drop in output voltage of the upstream oxygen sensor, which serves to perform an air-fuel ratio control, due to functional deterioration, the upstream oxygen sensor provides an output indicating an air-fuel ratio leaner than an actual air-fuel ratio (see FIG. 13), and, consequently, a feedback control is exercised so as to enrich the fuel mixture. Then, the downstream oxygen sensor provides an output indicating an air-fuel ratio of enriched fuel mixture. Consequently, as seen in FIG. 7, when the upstream oxygen sensor is functionally normal, i.e. has not functionally deteriorated, its output voltage, which must be at a low level so as to indicate an air-fuel ratio of lean fuel mixture, as shown by a broken line, shifts to a higher level that indicates an air-fuel ratio of rich fuel mixture, as shown by a solid line, due to an actual enrichment of fuel mixture resulting from a drop in output voltage of the upstream oxygen sensor. This results in reducing the number of reversal or reversal frequency of output voltage with respect to the normal threshold voltage, and decreasing the frequency ratio (Nb/Na) of reversals of an output of the downstream oxygen sensor to reversals of an output of the upstream oxygen sensor, so as to easily induce such an erroneous judgment that there is no deterioration of a catalyst, in spite of an actual and considerable deterioration.
On the one hand, the delay of response accompanies functional deterioration of the oxygen sensor, as shown in FIG. 14, as progressive deterioration occurs in the oxygen sensor and there is an accompanying gradual delay in response. For example, when a fuel mixture changes from rich to lean, the oxygen sensor gradually lowers its output from the normal voltage as shown by a solid line to a deteriorated voltage as shown by a broken line. If in fact such a delay in response occurs, when, for instance, the oxygen sensor provides an output changing from high level to low level accompanying a change in air-fuel ratio from rich to lean, a change occurs in air-fuel ratio before the oxygen sensor has completely changed its output from high level to low level, so that the oxygen sensor must reverse its output during the change in level of the output. This leads to reducing the amplitude of output of the oxygen sensor. As a result, although the normal output has an amplitude sufficiently large to cause a specified number of reverses with respect to the normal threshold voltage of reverse (RTV), for example, when the downstream oxygen sensor shows a delay in response due to its own deterioration, as shown by a broken line in FIG. 9, however, the output inclines towards the rich side as the result of a reduction in amplitude when the downstream oxygen sensor has deteriorated, as shown by a solid line. This leads to reducing the number of reversals of the output with respect to the normal threshold voltage of reverse (RTV) by the inclination to the rich side. As a result, the reversal frequency ratio (Nb/Na) is lowered, and, consequently, there is easily induced the erroneous judgment that no deterioration has occurred in the catalyst, in spite of an actual and considerable deterioration.
In the detection of deterioration of a catalyst on the basis of outputs from the oxygen sensors disposed in an exhaust line upstream and downstream from the catalytic converter, a great influence is given by deterioration of the oxygen sensor directly concerned with the detection of deterioration. However, there has not been provided a technological innovation from the standpoint of the oxygen sensor deterioration, and, consequently, it is obvious that an improvement must be made from the standpoint of accuracy and/or reliability in the detection of deterioration of the catalyst.